3 Gun/DMR rifle build

[DpdG]

I need help building a lighter weight 3 gun/SPR style 5.56/223 AR.

I’m looking to do 18”/rifle gas with rifle buffer, although I have lowers available with the full gamut of carbine/A5/rifle receiver extensions. I want to stay with full weight m16 BCG, fixed gas block and factory brass case (no steel), both 223 and 5.56 in primarily 55/77gr variety. The full weight/fixed gas block part seems outside the normal for most 3 gun types, and as a result I’m struggling with finding gas port sizing info for 18” barrels. Research so far has shown people reporting ports from .089 to .108, but very little is said about BCG/buffer weight or adjustable gas blocks. While I want reliable, I would prefer to stay on the conservative end of gas drive.

My thought was that .093 as Colt spec for a 20” SP1 was a starting place and given 2” less barrel after the port, the 18” will likely need a bit more port size, but how much more I can’t seem to figure out. Anyone able to help?

[rjohnson4405]

Never mind, wrong data :)

[mmc45414]

Can't offer anything specific on the port, just chiming in that I have an 18" gun I have been using for 3 gun and am happy with what I have. It is a little different than what you describe because I have a carbine stock. This helps at the small time local matches I have been shooting, because they have been creating stages with some contorted positions.

But I have been very happy with the 18" with the rifle length gas system. I have a new handguard that will cover the gas block on order with LaRue (since March...) to replace the cheese grater, after that change I will be all set (there have been a few barricades where the big handguard has been a little clunky to get through.

80697

[Dave J]

The Crane Mk-12 SPR had a port spec of .0995, with an 18” barrel and rifle length gas. Somewhere between .0995 and .100” is probably the answer, depending on what ammo you want it to run. The MK-12 was designed around Mk-262, which is pretty hot. I have an 18” upper similarly ported that sits on a A5 lower, and it’s reliable and a soft shooter, but it doesn’t get along well with underpowered ammo.

ETA: forgot to add that I’d originally paired that upper with a rifle buffer lower prior to switching it to an A5. Results about the same, although the rifle buffer was a bit less tolerant of weak ammo than the A5H2.

[SecondsCount]

I have 18", rifle length gas system barrels from Faxon and White Oak. Either one runs fine with an M16 carrier, using a lower with a standard carbine buffer system or a rifle buffer system.

I do have an adjustable block on the WOA and have turned it down to make it shoot a little softer off the bench. The Faxon barrel has a standard block.

[SecondsCount]

Double tap

[OlongJohnson]

But I have been very happy with the 18" with the rifle length gas system. I have a new handguard that will cover the gas block on order with LaRue (since March...) to replace the cheese grater, after that change I will be all set (there have been a few barricades where the big handguard has been a little clunky to get through.

Primary Arms had some exceptionally good pricing on BCM MCMR handguards over the weekend.

[mmc45414]

Primary Arms had some exceptionally good pricing on BCM MCMR handguards over the weekend.

When I said I circled the drain a little I meant to say I went a little batshit. At this point I have the LaRue LAT on 3/4, I can wait them out for for number four.

ETA: I think I confused threads. I mean that I went through all my ARs and I want it to match.

[pastaslinger]

I'd probably go with something like this:
Bcm mcmr or centurion cmr 15" rail
Criterion core 16" mid length or 18" rifle length barrel, if you don't want to wait then perhaps WOA or something else
Whatever quality bcg and upper receiver
Muzzle brake/comp of your choice

Quality lower of your choice
Vltor A5 system with a5h2 buffer
Trigger of your choice
Magpul SL stock
Magpul K2 grip

The Magpul furniture is a nice balance of weight and function. The rails are strong but light. The big thing to think about is the barrel, since it is going to swing a bit more with an 18" especially with too heavy a profile.

[03RN]

80826

I don't know off the top of my head what the port size is but this green mountain barrel has been very reliable and accurate.

[DpdG]

I'd probably go with something like this:
Bcm mcmr or centurion cmr 15" rail
Criterion core 16" mid length or 18" rifle length barrel, if you don't want to wait then perhaps WOA or something else
Whatever quality bcg and upper receiver
Muzzle brake/comp of your choice

Quality lower of your choice
Vltor A5 system with a5h2 buffer
Trigger of your choice
Magpul SL stock
Magpul K2 grip

The Magpul furniture is a nice balance of weight and function. The rails are strong but light. The big thing to think about is the barrel, since it is going to swing a bit more with an 18" especially with too heavy a profile.

This is very close to what I was thinking- Criterion Core 18” with matched/head spaced BCG, comp, either rifle or A5 buffer assembly/stock, and some sort of lighter weight rail. There is a dumb part of me that wants to put a 15” PRI gen3 rail on so it looks like a mix of Mk12 mod O and mod H (without swan rail). Unfortunately that would add about 6oz to the front end of the gun.

[jbrimlow]

I've been kicking some ideas like this around a lot. I'll spare you the tinker-heavy bits, but I'd suggest a heavier stock, preferably one with compartments in it that you can add weight to. You're going to throw an LPVO on there, plus a loaded 30+ round mag, and maybe a bipod. If you're going to run-n-gun with it, balance is really important. Way more so than weight. I also find that balance makes shooting offhand (standing unsupported) a lot less sucky.

[DpdG]

Completely agree on balance- I normally compete in 2/3 gun with either a very light AR with rds/mag (5.5lbs+ ammo) or a Spuhr’d out G3 with lvpo (12.5lbs+ ammo). Swing and front heaviness are very much a thing, hence trying to keep front end weight down. The Criterion Core should help, as it’s continuous taper to .625 gas block.

[jbrimlow]

Good call on that barrel profile. Way more weight sensible than the hybrid I ended up going with, but I had other concerns.

I echo the MCMR suggestion, unless you feel like springing for a Smoke Composites Carbon Fiber handguard.

Re: stocks, I think the best bet is a Magpul STR, Magpul ACS, or a SOPMOD. Basically, something with compartments for you to add weight to get the balance where you want it once you've set the length you want.

Just as an aside I ended up going with this stock: http://dissidentarms.com/index.php/product/daas/ for adjustability (and since it's almost certainly made by XLR, it works with their weights and monopod kit) so I can bulk up the rear end to make it swing right. It's super comfy, but it's carbine buffers only.